Concomitant with the reduction in size and weight of electronic products, higher precision in patterning of printed circuit boards has been required. Since flexible film substrates can be bent, which enables three-dimensional wiring, they are suitable for reduction in size of electronic products; hence, the demand for flexible film substrates is increasing. In TAB (tape automated bonding) techniques which are used for connecting ICs to liquid crystal display panels, by processing a relatively narrow, polyimide film substrate, a significantly excellent fine pattern as a resin substrate can be obtained. However, the development of micro-fabrication is practically reaching its limits. In order to evaluate miniaturization, an index represented by the line width and the space between lines, and an index represented by the position of the pattern on the substrate are used. With respect to the line width and the space between lines, further miniaturization may be possible. The latter index, i.e., the dimensional accuracy, relates to the alignment accuracy between a circuit board pattern and electrode pads when the circuit board and electronic components, such as ICs, are connected to each other, and as the pitch between electrode pads of ICs is further narrowed and the number of pins is increased, it becomes difficult to meet the required accuracy. That is, when an IC having more than 400 to 1,000 pins is connected to a circuit pattern, extremely high dimensional accuracy is required to align all the pins with fine electrode pads of the circuit pattern with a pitch of 40 μm or less and preferably 30 μm or less.
It is, in particular, difficult to improve the dimensional accuracy of flexible film substrates. In the circuit board fabrication, heat treatment processes, such as drying and curing, and wet processes, such as etching and development, are performed, and hence the flexible film is repeatedly subjected to expansion and shrinkage. The hysteresis during the fabrication processes mentioned above causes dimensional change of the circuit pattern on the substrate. In addition, when a plurality of processes requires alignment, if expansion and shrinkage occur during such processes, misalignment occurs between patterns to be formed. The deformation of the flexible film due to expansion and shrinkage has a more serious influence on an FTC (Flexible Printing Circuit) in which a substrate having a relatively large work-size is processed. In addition, dimensional change and misalignment are also caused by external forces, such as tension and torsion, and, in particular, when a thin substrate is used to increase flexibility, adequate care must be taken.
In addition, a proposal (International Publication No. WO 03/009657 pamphlet) for forming a circuit board has been made which is performed by adhering a flexible film onto a reinforcing plate with an organic layer interposed therebetween to maintain the dimensional accuracy, forming a very fine circuit pattern, and then peeling away the flexible film from the reinforcing plate. However, it has not been clearly disclosed that when the flexible film is peeled away from the reinforcing plate, by using a particular structure and method, the accuracy of the flexible film and that of the circuit pattern provided thereon are maintained, and that the peeling is performed while curling, bending and the like of the film is being prevented.
In addition, in peeling a flexible film in the past, in general, a rigid substrate was formed into a final product, and the flexible film was used as a protective film. Accordingly, the quality of the flexible film after peeled away has not been particularly considered, and hence attention has been primarily paid to reliable peeling of the flexible film. In addition, although some flexible products are formed after peeled away from rigid substrates, in this case, efficiency of peeling operation is primarily emphasized, or a method is used in which the product is bent in order to decrease a peeling force. Hence, the concept has not been considered at all in which peeling is performed while the flatness and dimensional accuracy of a flexible film are being maintained (for example, peeling is performed without causing a strain of approximately several hundred micrometers).
On the other hand, in recent years, a proposal has been made in which a very fine pattern is formed by adhering a flexible film to a reinforcing plate in order to maintain the dimensional accuracy. Since a circuit pattern of a flexible film substrate is used after the flexible film is peeled away from a reinforcing plate, the change in dimension of the circuit pattern generated when it is peeled away from the reinforcing substrate is preferably reduced to several tens micrometers or less. Hence, a peeling method has been desired in which a stress applied to a flexible film for peeling is decreased as small as possible.
As a method for peeling a flexible film from a rigid substrate, methods have been disclosed in which a flexible film is peeled away while a rigid plate is being held. As particular examples, for example, there may be mentioned methods for peeling a flexible film by lifting up an end portion thereof while the end portion of the flexible film is held (for example, see Japanese Unexamined Patent Application Publication No. 7-315682), while a pressure-sensitive adhesive tape is pressed onto the surface of the flexible film (for example, see Japanese Unexamined Patent Application Publication No. 5-319675), and while a peel angle formed between the rigid substrate and the flexible film is maintained obtuse (for example, see Japanese Unexamined Patent Application Publication No. 2002-104726). In addition, for example, there may also be mentioned a method (for example, see Japanese Unexamined Patent Application Publication No. 7-215577) in which a flexible film is peeled away from a peeling roller using a scraper after the flexible film is transferred onto the peeling roller. However, in all the methods described above, the flexible film used as a protective film is peeled away from a product, and a method for peeling a flexible film having a very fine pattern formed thereon without degrading the dimensional accuracy and flatness has not been described at all.